Cytochrome oxidase is the terminal enzyme of cellular respiration. It functions to catalyze the reduction of dioxygen to water, coupling the resultant energy through the cytochrome chain into the oxidative phosphorylation process. Its metabolic role is essential to life. In spite of being one of the most studied enzyme systems in existence, it also remains one of the most enigmatic. By studying well chosen model compounds we can increase our understanding of this most important enzyme. The goals of this investigation are: a) to prepare Fe/Cu complexes which mimic the active site in cytochrome oxidase, b) to examine the physical and chemical properties of these complexes, and c) to examine the mechanism of their reaction with dioxygen. The types of complexes proposed are derived from synthetic porphyrins having a second tetradentate coordination site "built" into the molecule. Iron/copper mixed-metal complexes of these ligands will be prepared and studied by the same series of methods and techniques that have been employed to study the natural enzyme. These will include: EPR, electrochemistry, magnetic susceptibility, optical spectroscopy, Mossbauer spectroscopy, ligand binding studies and simple reaction rate studies. The results of the model studies will be used in an effort to try to better understand similar data which exist for the enzyme.